As described in an article entitled "Stereophonic Sound For Television" appearing in the I.E.E.E. Spectrum for September 1982, in many parts of Japan and West Germany, the television channels provide stereophonic sound in conjunction with the television programs. In the United States, however, only monophonic sound is provided. However, it is contemplated in the near future that television broadcasters in the United States will begin transmitting stereophonic sound for the audio portion of selected television programs. Several transmission systems for stereophonic television sound have been proposed, including a proposal from the Zenith Radio Corporation of Glenview, Ill. and several others.
A feature of the proposed stereophonic system is that the stereophonic sound signals can be received and reproduced monophonically on existing television receivers. In the future, however, it is contemplated that television receivers will be marketed with the necessary dual channels and speakers for self-contained stereophonic reproduction.
The Zenith stereophonic system is described, for example, in U.S. Pat. No. 4,339,772 which issued July 13, 1982, and which is assigned to Zenith. As described in that patent, under present television broadcasting standards, a band of frequencies approximately 80 KHz wide is designated within each 6 MHz television channel for the transmission of the audio component of the television signal.
Within this band of frequencies, a radio frequency main audio carrier is frequency modulated by an audio base band signal for producing a monaural transmission signal. The transmitted monaural audio signal may be received by a television receiver which converts the radio frequency audio carrier signal into a signal having a frequency centered at 4.5 MHz. The converted 4.5 MHz sound carrier is then processed by a frequency modulation detector to reproduce the monaural audio signal. In this regard, it has been recognized that the audio bandwidth of present-day television channels is presently being under utilized, thus affording the opportunity for the transmission of a substantial amount of audio information in addition to the conventional monaural signal.
As also pointed out in U.S. Pat. No. 4,339,772, in addition to transmitting stereophonic sound components on the main aural carrier of a transmitted television signal, it is also desirable to transmit additional information so as to utilize more completely the available audio bandwidth within the television channel. For example, the transmission of a second language audio signal would enable a viewer selectively to operate a television receiver for reproducing the audio signals associated with the transmitted stereophonic information or, alternatively, reproduce the audio signals associated with the transmitted second language information. Other examples of such additional information include electronic news gathering (ENG) signals and telemetry signals, which may be utilized by television broadcasters for their own private use.
An objective of the present invention is to provide a simple and inexpensive split-sound receiver for use in such a system, in conjunction with a conventional television receiver, whereby the picture content of a transmitted television signal may be reproduced by the conventional television receiver, with the sound muted, and whereby first and second language audio signals may be selectively reproduced by the receiver of the invention. The receiver of the invention includes a self-contained antenna for receiving radio frequency signals from the television receiver, and a radio frequency channel for passing signals corresponding only to a single channel of the television receiver so as to maintain the simplicity of the receiver, and to reduce its cost to an absolute minimum.